I agree with others that in most cases, you're better off sticking with the ISO 8601 definitions. But, just in case you're interested, Date::Manip defaults to ISO 8601, but you can specify other definitions of week using some of the config variables that are built in. For example:
- FirstDay can be used to specify a different day of the week as the first day
- Jan1Week1 can be used to say that Jan 1 is in the first week of the year
Most of the time I would discourage using these... but they are there if you need them.